I really like the Wizard of Oz idea you have, and I get a little bit of an Alice in wonderland vibe as well.
It sounds like you want Magic to be kinda mischievous and what first came to mind was having excess magic be like a wind that's opalescent and smells like cupcakes or something and have it "cast a spell." sort of like a sorcerers Wild Magic Surge but it's from nature. You can have it appear in every session as often as you like, and by having it be something they can see and smell it let's them know that now something funny/curious/weird is going to happen.
my first thought would be to have it appear when they're traveling and having it do funny things. "Your walking through the woods and you smell cupcakes, and the smell is gone." Then have trees making fun of them as they walk by. Basically the "Magic Wind" cast Speak with Plants on the party.
Or have all the flowers around them start to sneeze, and it causes the Sleep spell to be cast.
Have their food taste like a different kind of food, apples taste like oranges.
Have them random grow and shrink like in the Enlarge/Reduce spell
Have their utensils spring to life like in Beauty and the Beast
Have Speaks with Animals cast on them and all of a sudden squirrels are hitting on the wizard
Or anything else that feels whimsical but benign. The only guide I would follow is to make sure that it affects the whole party at once (People might feel left out, or picked on), and that it doesn't cause any direct harm to anyone.
it helps very much!
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